use CI actions/checkout v2

Author: dekellum

.github/workflows/ci.yml

@@ -43,7 +43,7 @@ jobs:
 
     steps:
       - name: Checkout
-        uses: actions/checkout@v1
+        uses: actions/checkout@v2
 
       - name: Install Rust (${{ matrix.rust }})
         uses: actions-rs/toolchain@v1